1. Field of the Invention
The present invention pertains to a radiographic image capturing system, a program storage medium, and a method, and particularly relates to a radiographic image capturing system that captures radiographic images using a grid for removing scatter radiation caused by a subject, a storage medium in which is stored a program that is executed in the radiographic image capturing system, and a method.
2. Description of the Related Art
In recent years, radiation detectors such as flat panel detectors (FPD), in which a radiation-sensitive layer is placed on a thin-film transistor (TFT) active matrix substrate and which can convert radiation directly into digital data, have been put into practical use. Radiographic image capturing devices that use these radiation detectors to capture radiographic images expressed by applied radiation have also been put into practical use. Types of radiation detectors used in these radiographic image capturing devices include indirect conversion radiation detectors, which convert radiation into light by means of a scintillator and thereafter convert the light into an electric charge by means of a semiconductor layer such as a photodiode, and direct conversion radiation detectors, which convert radiation into an electric charge by means of a semiconductor layer such as amorphous selenium. In each type, there exist various materials that can be used for the semiconductor layer.
In this type of radiographic image capturing device, a grid in which a material whose radiation absorption rate is high and a material whose radiation absorption rate is low are arranged side by side parallel to each other and alternating at regular intervals is used to remove scatter radiation scattered by the subject. There have been cases where, due to the difference between the spatial frequency (the spacing, with respect to the array direction, of the material whose radiation absorption rate is high) that the grid has and the spatial sampling period (detection pixel spacing) of the radiation detector, moiré fringes arise in the image obtained by the radiation detector.
That is, the Nyquist frequency fN [lines/cm] of a radiation detector with a pixel spacing Δ [cm] is expressed by the following expression (1).
                              f          N                =                  1                      2            ×            Δ                                              (        1        )            
For example, if the pixel spacing Δ is 150 [μm], the Nyquist frequency is 33.3 [lines/cm]. Further, in a grid in which the number of lines, per 1 cm in the array direction, of the material whose radiation absorption rate is high (hereinafter called “the radiation absorbing material”) is fG [lines/cm], the number of lines fG′ [lines/cm] per 1 cm of the radiation absorbing material in the radiographic image is expressed by the following expression (2), and moiré fringes are generated in the spatial frequency (hereinafter also simply called “frequency”) of fG′ in a case in which the grid has been placed with respect to a radiation detector in such a way as to align the array direction of the radiation absorbing material with the pixel array direction of the radiation detector.fG′=min(fG−2NGLN,2(NG+1)fN−fG)  (2)
NG in expression (2) is an integer equal to or greater than 0 (zero) and is expressed by the following expression (3). Here, the brackets in expression (3) are symbols meaning the decimal point and all numbers thereafter are discarded.
                              N          G                =                  [                                    f              G                                      2              ×                              f                N                                              ]                                    (        3        )            
As a technology that can be applied for preventing the generation of moiré fringes, Japanese Patent No. 4,500,400 discloses an image acquiring device that acquires an image by two-dimensionally sampling X-rays that have passed through a subject. The image acquiring device is equipped with an image acquiring unit that acquires an image by two-dimensionally sampling X-rays, a scatter radiation removing grid that is placed between the image acquiring unit and the subject and has a spacing that is from 30% to 40% of a sampling frequency sampled by the image acquiring unit, an image processing unit that removes grid lines caused by the scatter radiation removing grid, and a selection unit that automatically selects whether or not to remove the grid lines with the image processing unit based on information on an imaging site.
Expression (2) may appear complicated, but as shown in FIG. 20, it simply means that the spatial frequency fG′ takes values that draw a line that is reflected (folded) at the vertical lines of the Nyquist frequency fN and f=0, which are serving as walls.
For example, NG with respect to a grid in which the number of lines fG is 60 [lines/cm] is 0 (zero), and fG′ is 6.66 [lines/cm] as shown in the following expression (4).fG′=min(fG,2fN−fG)=min(60,2×33.33−60)=min(60,6.66)=6.66  (4)
An upper limit value fμ of the spatial frequency of human body signals obtained by Fourier transforming, and mapping in a frequency space, image information (data) representing a radiographic image expressed by radiation that has passed through a human body is usually a low frequency and, although it depends on the site being imaged, it is estimated that there is virtually no information in signals in a frequency band of 20 [lines/cm] or greater. Supposing that μ (e.g., μ=0.6) represents the ratio of this upper limit value fμ with respect to the Nyquist frequency fN, when it satisfies the following expression (6), human body signals are not impaired much even if signals of a frequency around fG′ are removed by image processing.fG′>μfN=fμ (0<μ<1)  (5)that is,(2NG+1−μ)fN<fG<(2NG+1+μ)fN  (6)
As shown in FIG. 20 as an example, usually a direct conversion radiation detector has a sensitivity in a frequency across 200 [lines/cm] or more, and in an indirect conversion radiation detector, the scintillator has a sensitivity up to about f0≈80 [lines/cm] (i.e., the signal strength is greater than 0 (zero) in a frequency up to about f0≈80 [lines/cm]). For that reason, it is necessary for a direct conversion radiation detector to always satisfy expression (5), and for an indirect conversion radiation detector, necessary that the harmonic expressed by the following expression (e.g., in the case of N=2, f2G′ obtained by solving fG as f2G=2fG in expression (2)) satisfies expression (5).NfG<f0(N>1)
In an indirect conversion radiation detector, if a grid with a relatively large number of lines (e.g., fG≈2fN) is chosen, the harmonic can be made into a high frequency with no sensitivity, but the folding frequency fG′ will position in a low frequency (fG′≈0<μfN) that cannot be ignored. If a grid with a relatively small number of lines (e.g., fG≈fN) is chosen, the folding frequency fG′ will be a high frequency, but the harmonic f2G′ comes in a frequency band to which the scintillator has a sensitivity, so the folding frequency f2G′ will position in a low frequency (f2G′≈0<μfN) that cannot be ignored.
With respect to this, there is also a technique that chooses a grid or a pixel spacing in which the harmonic satisfies NfG′>fμ′ as described in Japanese Patent No. 4,500,400, but adopting this technique restricts the design of the pixel spacing and so forth.